The present invention relates to an apparatus for measuring a variation in a circumference of a body part and method for plethysmography.
Plethysmography is a procedure which has been known for some time now and which is used for determining macro- and microvascular parameters in the extremities, such as the venous capacity, the venous reflux, the venous elasticity, the venous outflow rate, the material blood flow and the capillary filtration rate. In general, plethysmography allows qualitative and quantitative statements to be made concerning the state and function of the macro- and microvascular circulation in an extremity of a patient.
Plethysmography can be carried out in a very wide variety of ways, for example as water plethysmography, air plethysmography, impedance plethysmography, capacitance plethysmography, induction plethysmography or strain gauge plethysmography. These procedures make use of different physical phenomena for determining the state of the blood vessels in a body part. The present invention applies equally to venous compression plethysmography, where blood flow in the veins is occluded by means of a cuff, and to dynamic or functional plethysmography, where the blood flow in the body""s blood vessels is influenced by exercise or maneuvers of the body.
In the prior art a form of strain gauge plethysmography concerning circumference changes in an extremity is disclosed in U.S. Pat. No. 3,847,142 (issued to Williams, Jr. et al. on Nov. 12, 1974). The apparatus described therein comprises a first cuff for the wrist to occlude the blood flow to the hand, a second cuff to be put arround the upper arm and a rubber strain gauge filled with mercury, to be put arround the largest part of the forearm to detect changes in its circumference. The strain gauge serves as one arm of a Wheatstone bridge circuit, the output voltage of which varies in a linear fashion according to the length of the strain gauge. The apparatus needs to be calibrated before use and requires two cuffs plus strain gauge; therefore it is rather difficult to operate. It also requires a strain gauge filled with mercury, which is relatively expensive and can be dangerous for the operating person in case of an accident or defect. For a patient suffering from venous disease, it would be desirable to have an apparatus for use at home, to determine the venous parameters himself. Such an apparatus should be easy to handle but at the same time reliably provide accurate measured values. It should be relatively low in costs and should not contain substances which could threaten the health of the patient or any other operating person.
It is therefore the object of the present invention to make available an apparatus for measuring a variation in a circumference of a body part and a method for plethysmography which does not have the disadvantages of the prior art.
According to the invention, there is provided an apparatus for measuring a variation in a circumference of a body part comprising an elongated force transmission element, an elongated support element, a casing, and a displacement measuring device, said force transmission element having a first end and a second end and not being expandable longitudinally, said support element having a first end and a second end and being expandable longitudinally, said force transmission element being slidingly arranged on said support element, said displacement measuring device having force connecting means for being connectable or connected to said first end of said force transmission element and being moveable relative to said casing, and having support connecting means for being connectable or connected to said first end and to said second end of said support element, and further having fixing means for being connectable or connected to said second end of said force transmission element and being fixed relative to said casing, said displacement measuring device having measurement means for measuring a relative movement between said force connecting means and said casing.
In the above paragraph, several elements are connectable or connected to other elements, because it is a question of convenience, costs and possibly other factors whether, for example, the force connecting means are connected to the first end of the force transmission element from the beginning, or whether this connection is only made by a person applying the inventive apparatus to a patient before beginning the measurement process.
It should also be noted, that the apparatus according to the invention measures a variation in a circumference of a body part, which requires a comparatively high resolution in terms of time e.g. measuring a value every 100 ms.
The present invention simplifies the procedure of plethysmography by measuring a variation in the circumference of a body part. An adjustment of the apparatus is not generally necessary, because the force transmission element is particularly adapted to the size of the body part of the patient which has to be examined.
Another distinct advantage of the present invention is, that it does not require an expensive strain gauge filled with mercury or a similar substance and therefore represents no danger to the health of the patient or the operating person. The force transmission element according to the invention is preferably a yarn made of polyester material which is relatively low in production costs, does not contain any harmful substances and is more stable than a conventional rubber strain gauge.
In a preferred embodiment of the invention said fixing means include adjustment means for varying the effective length of said force transmission element around said body part by either displacing said fixing means relative to said casing or by adding length to said force transmission element between said force connecting means and said fixing means. In case an adjustment of the apparatus has to be performed, the operator, for example the patient himself, a doctor or a nurse, successively adjusts the effective length of the force transmission element manually. The adjustment can be accomplished by a screw mechanism or a clamping mechanism. Both mechanisms may be applied to both, the displacement of the fixing means relative to the casing and to the addition of length to the force transmission element.
In another advantageous embodiment of the invention, said displacement measuring means include indicator means for indicating the correct position of said force connecting means in relation to said casing of said displacement measuring device by issuing an optical or acoustical signal.
Yet another preferred embodiment of the invention concerns a displacement measuring device, which includes inductive means for measuring said relative movement between said force connecting means and said casing.
The present invention also includes a system for venous compression plethysmography with an apparatus according to claim 1 and a cuff whose internal diameter can be varied in order to occlude blood flow in a body part.
The invention also comprises a method for plethysmography using an apparatus as described above, comprising the following steps:
a) arranging said apparatus on said body part by positioning said force transmission element and said support element around said body part, such that said first end of said force transmission element is connected to said force connecting means, that said second end of said force transmission element is connected to said fixing means, and that said first and second end of said support element are connected to said support connecting means, and
b) registering by means of said measurement means of said displacement measuring device any relative movement between said force connecting means and said casing.
It is also conceivable, that between steps a) and b) of the method according to claims 8-9, a further step of adjusting by means of adjustment means the effective length of said force transmission element is integrated.